1. Field of the Invention
The present invention relates to the field of downhole fluid analysis in boreholes penetrating the earth. More particularly, the present invention relates to a method and apparatus for estimating fluid density, viscosity, and other parameters using a flexural mechanical resonator downhole in a borehole during monitoring while drilling or wire line operations.
2. Description of the Related Art
There is considerable interest in obtaining density and viscosity for formation fluids downhole at reservoir conditions of extreme temperature and pressure during formation sampling, production, or drilling.
One type of instrument for measuring the density and viscosity of a formation fluid is a flexural mechanical resonator such as a tuning fork. The tuning fork is immersed in a fluid downhole and electrically excited at multiple frequencies. The interaction of the tuning fork with the fluid will introduce additional complex electrical impedance that can be measured in an electrical circuit. It has been shown that this complex electrical impedance can be represented by the sum of two terms: one that is proportional to liquid density and a second one that is proportional to the square root of the viscosity density product.
Unfortunately, conventional flexural mechanical resonators when used downhole can present several problems. For example, a tuning fork may be prone to breakage if not properly protected. Surface contamination of a flexural mechanical resonator due to being disposed downhole may cause low repeatability in resonator measurements. Flexural mechanical resonators, which use permanent magnets, may be prone to attract magnetic particles. These magnetic particles cannot be removed downhole and can affect the accuracy of measurements.
It is important for flexural mechanical resonators to function properly downhole because it can be very expensive in time and equipment if they fail and have to be extracted from the borehole, repaired or replaced, and then sent back down the borehole. It would be well received in the drilling industry if flexural mechanical resonators could be made more robust to survive the extreme environment downhole.